Wire bonding is one method for semiconductor assembly. The wire bonding flow includes three primary steps, die attach, wire bond and encapsulation. The wire bond process electrically connects the bond pads on the semiconductor die to pads, leads or pins on the workpiece. The wire bond machine essentially welds a fine metal wire between each bond pad on the die and the appropriate pad, lead or pin on the workpiece.
During the assembly of wire bound semiconductor devices, the wire bonding can be misaligned (or shifted) from the center of the bond pads. Die having misaligned wire bonds may still pass final electrical test when the degree of misalignment is such that the respective bond pads or their conductive material thereon approach one another, but do not touch. However, such die can become potential (latent) shorts. For example, if die having substantially misaligned wire bonds (e.g. wire bonds near the edge of the bond pads) receives burn-in stress or is used in the field, the misaligned wire bonds can become further displaced, and/or metal on the bond pad can become displaced, sometimes sufficient to create a bridge that shorts between adjacent bond pads.
Microscope inspection of the wire bonding after burn-in (e.g., 168 hours at 125° C.) can be performed (e.g., 40× optical microscope) in an attempt to screen out latent shorts. However, this method is not compatible with mass production, and has trouble detecting misaligned bond wires, particularly if the bond pads and bond pad pitch are small. Thus, microscope inspection may not detect all wire bond shorts between adjacent bond pads, or latent shorts. Even electrical test after burn-in will not detect latent shorts. Accordingly, die having shorts after burn-in or latent shorts between bond pads can pass final test and thus be sold to customers, which can manifest as failures first identified by the customers.